Enable mixed approx_count_distinct aggregation#4823
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michaelmitchell-bit wants to merge 2 commits into
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Enable mixed approx_count_distinct aggregation#4823michaelmitchell-bit wants to merge 2 commits into
michaelmitchell-bit wants to merge 2 commits into
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Add native support for Spark's approx_count_distinct, a faithful port of Spark's HyperLogLogPlusPlus / HyperLogLogPlusPlusHelper. Each non-null input is hashed with Comet's Spark-compatible XxHash64 (seed 42, floats normalized first), and the HyperLogLog++ registers are stored in Spark's exact packed-Long buffer layout (10 six-bit registers per word). The cardinality is estimated with the same linear-counting and bias-correction tables Spark uses, so results are bit-identical to Spark and the partial-aggregation state matches Spark's aggBufferSchema. Includes a vectorized GroupsAccumulator, SQL file tests comparing against Spark across a range of cardinalities, native unit tests, benchmark coverage, and documentation updates.
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Important
Draft/stacked on #4819. Please do not review this independently until #4819 lands; after that I will rebase/update this PR so the reviewer diff is only the mixed partial/final follow-up.
Which issue does this PR close?
Closes #4820.
Depends on #4819.
Rationale for this change
After #4819,
approx_count_distinctstores HyperLogLogPlusPlus buffers in the same packed Long layout as Spark. That makes the partial/final aggregate state compatible across Spark and Comet, so Comet can safely enable mixed partial/final execution for this aggregate.What changes are included in this PR?
CometApproxCountDistinct.count(DISTINCT ...)+approx_count_distinct(...)with Comet partial/merge stages feeding a Spark final stage.approx_count_distinctthrough Spark's PartialMerge aggregate path.How are these changes tested?
make core./mvnw test -Dtest=none -Dsuites="org.apache.comet.exec.CometAggregateSuite approx_count_distinct partial merge" -Dscalastyle.skip=true./mvnw test -Dtest=none -Dsuites="org.apache.comet.rules.CometExecRuleSuite approx_count_distinct" -Dscalastyle.skip=true./mvnw test -Dtest=none -Dsuites="org.apache.comet.CometSqlFileTestSuite partial_merge" -Dscalastyle.skip=true